Synthetic cephalotaxine esters having antileukemic activity

ABSTRACT

Six acyl esters of cephalotaxine have been synthesized by ordinary and standard procedures, and all have demonstrated chemotherapeutic activity against leukemia in animals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a group of synthetic esters of cephalotaxineand the use of these esters as chemotherapeutic agents for the remissionof leukemia in animals.

2. Description of the Prior Art

Among the alkaloids which have been isolated from Cephalotaxusharringtonia plant material are cephalotaxine and a number of its esters[Powell et al., Tetrahedron Lett. 4081 (1969); Powell et al.,Tetrahedron Lett. 815 (1970); Mikolajczak et al., Tetrahedron 28: 1995(1972); U.S. Pat. No. 3,793,454; and U.S. Pat. No. 3,870,727]. Thoughcephalotaxine itself is inactive, some of its esters which are derivedfrom relatively complex dicarboxylic acid moieties have been found toexhibit significant activity against experimental leukemia systems[Powell et al., J. Pharm. Sci. 61(8): 1227-1230 (August 1972)]. Two ofthe esters, harringtonine and homoharringtonine, have been approved forpreclinical evaluation at the National Cancer Institute. However, plantmaterial from which to extract the active esters is in critically shortsupply.

Cephalotaxine has been synthesized [Weinreb et al., J. Am. Chem. Soc.97: 2503 (1975); Semmelhack et al., J. Am. Chem. Soc. 97: 2507 (1975);and Weinreb and Semmelhack, Acc. Chem. Res. 8: 158 (1975)] therebystimulating efforts to convert it to some of its active, naturallyoccurring esters by reaction with appropriate acid compounds. However,very unfavorable steric (and perhaps electronic) intereactions at thereaction sites of both the cephalotaxine and the acyl moiety precludedirect esterification [Mikolajczak et al., J. Pharm. Sci. 63: 1280(1974)]. By means of complicated and indirect routes, cephalotaxine hasbeen converted to the active esters, deoxyharringtonine [U.S. Pat. No.3,959,312; Mikolajczak et al., Tetrahedron Lett. 283 (1974); and Li etal., Hua Hsueh Hsueh Pao 33: 75 (1975)]; and harringtonine [Anonymous,K'o Hsueh T'ung Pao 20: 437 (1975); Chem. Abstr. 84: 105859Z (1976)].

SUMMARY OF THE INVENTION

We have now surprisingly found a group of synthetic acyl esters ofcephalotaxine which show activity against leukemia in animals, and whichcan be prepared from cephalotaxine by ordinary and standard proceduresbecause they are not subject to the severe steric requirements of theprior art compounds. These alkaloid compounds are characterized by thefollowing structural formula: ##STR1## where R is selected from thegroup consisting of ##STR2##

It is therefore an object of this invention to obtain from cephalotaxinesynthetic esters which have activity against leukemia.

Another object of the invention is to prepare these active cephalotaxineesters by ordinary and simple procedures.

It is also an object of the invention to administer the novel alkaloidcompounds to animals in order to cause remission of leukemia therein.

Other objects and advantages of the invention will become readilyapparent from the ensuing description.

DETAILED DESCRIPTION OF THE INVENTION

Cephalotaxine is characterized by the following structural formula:##STR3## where R equals H. We have found that by substituting selectedacyl groupings for the hydroxyl hydrogen, the chemotherapeuticallyinactive cephalotaxine can be converted to antileukemic alkaloids. Itwould appear from preliminary investigations that the activity ofcephalotaxine esters is attributable primarily to the (-)-cephalotaxineenantiomer. This is the enantiomer isolated from natural sources.Cephalotaxine produced synthetically, such as by one of the procedurestaught in the references discussed above, occurs as a racemic mixture.While it is preferred to use the (-)-isomer, either isolated from plantsor separated from a synthetic (+)-mixture, it is understood that the(+)-mixture itself would be a suitable starting material.

The acylating agents are preferably acid chlorides or anhydrides whichare commercially available, or else readily obtainable by conventionalmodification of available precursor compounds, such as the appropriateacid, ester, or half ester. Such modifications are illustrated in theexamples below.

Acylation and recovery procedures for obtaining the instant cephalotaxylesters do not in themselves constitute novelty within the instantinvention. However, these have been illustrated in detail in theaccompanying examples for each of the disclosed species. It isunderstood that certain obvious alterations and variations of theseprocedures, involving the reagents, proportions, solvent systems, andother reaction conditions and parameters may be made without alteringthe nature of the final products. For instance, we have found thatmethyl cephalotaxyl fumarate ##STR4## may be prepared with either methylhydrogen fumarate or maleic anhydride as the acylating agent, due toisomerization of the double bond in the anhydride.

In the examples which follow, anhydrous reagents, solvents, andsolutions of reactants were prepared by drying for at least 4 hours overtype 3A or 4A molecular sieve. Extracts of aqueous systems wereroutinely dried with MgSO₄. All purification steps were monitored bythin layer chromatography, and in most instances by infrared analysis(IR). All the isolated cephalotaxine esters were amorphous solids, andeach gave an IR, nuclear magnetic resonance, and mass spectrum (MS)consistent with its structure (see Table I for MS analysis).High-resolution mass spectral analyses were performed with a Nuclide12-90G spectrometer.

EXAMPLE 1 Preparation of ethyl cephalotaxyl oxalate ##STR5##

(-)-Cephalotaxine (5.0 g.) and 1.34 g. of pyridine in 20 ml. ofanhydrous CH₂ Cl₂ were cooled in an ice bath. Ethyl oxalyl chloride(2.38 g.) in 10 ml. of CH₂ Cl₂ was added dropwise over 1 hour. Stirringat 0° C. was continued 3 more hours and then at room temperatureovernight. The mixture was poured into 100 ml. of pH 7.0 phosphatebuffer solution, and the solution was extracted with CH₂ Cl₂.Evaporation of the CH₂ Cl₂ afforded pure ethyl cephalotaxyl oxalate. Theyield was 71% based on the (-)-cephalotaxine used.

EXAMPLE 2 Preparation of methyl cephalotaxyl fumarate ##STR6##

Fumaric acid (40.0 g.) in 150 ml. of anhydrous benzene and 75 ml. ofdioxane was treated with 18 ml. (25% excess over the amount needed toesterify one carboxyl group) of MeOH and 2 ml. of concentrated H₂ SO₄.The mixture was refluxed under a Dean-Stark trap until no more watercollected in the trap, about 4 hours. The solvent was concentrated to 25ml., water was added, and the mixture was extracted with diethyl ether.After evaporation of ether, the residue was separated in 3.0-g. batcheson a 2.5×35 cm. column of silica gel with 300 ml. of ethylacetate:benzene (10:90) followed by 400 ml. of ethyl acetate:benzene(20:80). Yield of methyl hydrogen fumarate was 15%.

Methyl hydrogen fumarate (3.22 g.) was treated overnight with stirringat room temperature with 10 ml. of oxalyl chloride. Excess oxalylchloride was evaporated in vacuo, and the residue (the acid chloride)was dissolved in 10 ml. of CH₂ Cl₂. A solution of 3.1 g. of(-)-cephalotaxine and 3 ml. of pyridine in 10 ml. of CH₂ Cl₂ was addedto the acid chloride slowly over 30 minutes., and the resulting solutionstirred overnight. The mixture was poured into 75 ml. of 5% Na₂ CO₃solution and extracted with CH₂ Cl₂. After evaporation of CH₂ Cl₂, theresidue was dissolved in diethyl ether:petroleum ether (75:25) and runthrough a 1×10 cm. neutral alumina (Woelm, grade III) column. The yieldof methyl cephalotaxyl fumarate was 68% based on the (-)-cephalotaxineused.

EXAMPLE 3 Preparation of methyl cephalotaxyl itaconate ##STR7##

Itaconic acid (248 g.) was treated with 246 ml. of anhydrous methanoland 4.0 ml. acetyl chloride at reflux for 20 minutes. Excess MeOH wasevaporated in vacuo and the residue was crystallized by adding 200 ml.of benzene, followed by 300 ml. of petroleum ether (b.p. 30°-60° C.) andchilling the solution to 0° C. The crystals were recrystallized frombenzene:petroleum ether (3:2) and gave a m.p. of 66°-69° C. The yield ofmethyl hydrogen itaconate was 42% (115 g.).

Methyl hydrogen itaconate (3.17 g.) was dissolved in anhydrous ether anddried over type 4A molecular sieve. The ether solution was removed fromthe sieve and all solvent removed in vacuo. The residue was treated neatwith 10 ml. of oxalyl chloride overnight at room temperature withstirring (magnetic). Excess oxalyl chloride was removed in vacuo and theacid chloride was dissolved in anhydrous CH₂ Cl₂ and cooled in an icebath. To this cold solution of the acid chloride was added dropwise asolution of 3.15 g. of (-)-cephalotaxine and 4 ml. of pyridine in 10 ml.of CH₂ Cl₂. The mixture was allowed to warm to room temperature and wasstirred overnight.

This mixture was poured into 50 ml. of 5% Na₂ CO₃ solution, and thesolution was extracted with CH₂ Cl₂. The crude product was dissolved indiethyl ether:CH₂ Cl₂ (2:1) and was run through a neutral alumina(Woelm, grade III) column of 1×10 cm. The yield of pure methylcephalotaxyl itaconate was 83% based on the (-)-cephalotaxine used.

EXAMPLE 4 Preparation of cephalotaxyl trans,trans-sorbate ##STR8##

trans,trans-Sorbic acid (2.24 g.) was treated with oxalyl chloride for 2hours. Excess oxalyl chloride was evaporated in vacuo. The residue wasdissolved in 10 ml. of CH₂ Cl₂ and cooled in an ice bath. A solution of3.1 g. of (-)-cephalotaxine and 3 ml. of pyridine in CH₂ Cl₂ (10 ml.)was added slowly (30 minutes) to the cold acid chloride solution and themixture stirred at room temperature overnight. The mixture was pouredinto 75 ml. of 5% Na₂ CO₃ solution and extracted with CH₂ Cl₂. The crudeproduct remaining after removal of CH₂ Cl₂ was purified bychromatography on a 1×10 cm. neutral alumina (Woelm, grade III) columnwith diethyl ether. This was followed by chromatography on a silica gelcolumn, 2.5×35 cm., with CH₂ Cl₂. The yield of cephalotaxyltrans,trans-sorbate was 36% based on (-)-cephalotaxine used.

EXAMPLE 5 Preparation of cephalotaxyl L-mandelate,trichloroethylcarbonate ester ##STR9##

L-Mandelic acid (10 g.), 6.91 g. of benzyl alcohol, and 75 mg. ofp-toluenesulfonic acid in 70 ml. of benzene was refluxed under aDean-Stark trap for a total of 28 hours on 4 consecutive days. The crudebenzyl ester was obtained by successively concentrating the liquor andcooling it to about 10° C. a number of times. The fractions meltingbetween 97° C. and 105° C. were combined, dissolved in CHCl₃ and washedtwice with 15-ml. portions of 5% aqueous NaHCO₃ solution. Afterevaporation of the CHCl₃, the product was recrystallized from benzene togive benzyl L-mandelate, m.p. 104°-106° C., yield 42%.

6.5 Grams of benzyl L-mandelate was dissolved in CH₂ Cl₂ (25 ml.) andwas treated dropwise with 5.90 g. of trichloroethoxycarbonyl chloride in15 ml. of CH₂ Cl₂. The mixture was stirred overnight at room temperatureand then was poured into 75 ml. of 5% Na₂ CO₃ solution and extractedwith CH₂ Cl₂. The crude product ##STR10## was purified by chromatographyof 2-g. batches on a 2.5×35 cm. silica gel column with 200 ml. ofbenzene:petroleum ether, 25:75, 200 ml. of 35:65 and enough of 45:55 tocompletely elute the desired ester. The ester was then crystallized fromdiethyl ether at 0° C. to give a 68% yield of thetrichloroethylcarbonate ester of benzyl L-mandelate, m.p. 96°-97° C.Hydrogenolysis of this ester (to remove the benzyl ester grouping) wasdone by hydrogenating 2-g. batches dissolved in 10 ml. oftetrahydrofuran with 200 mg. of 10% Pd/C until one molar equivalent ofH₂ was consumed. This gave a 97% yield of ##STR11##

4.1 Grams of above acid was treated at reflux for 1 hour with 15 ml. ofoxalyl chloride and the excess reagent removed in vacuo. The residue wasdissolved in CH₂ Cl₂ (20 ml.) and cooled in an ice bath. A solution of2.52 g. of (-)-cephalotaxine and 2 ml. of pyridine in 20 ml. of CH₂ Cl₂was added slowly. The mixture was allowed to warm to room temperatureand was then stirred overnight at room temperature. The mixture waspoured into 75 ml. of 5% Na₂ CO₃ solution which was then extracted withCH₂ Cl₂. After evaporation of the CH₂ Cl₂, the crude cephalotaxylL-mandelate, trichloroethylcarbonate ester was run through a 1×10 cm.neutral alumina (Woelm, grade III) column in 2-g. batches with diethylether. The yield was 91% based on (-)-cephalotaxine used.

EXAMPLE 6 Preparation of cephalotaxyl trichloroethylcarbonate ##STR12##

(-)-Cephalotaxine (2.40 g.) and 2 ml. of pyridine in 20 ml. of CH₂ Cl₂(anhydrous) was cooled in an ice bath. Then 1.80 g. oftrichloroethoxycarbonyl chloride in 5 ml. of CH₂ Cl₂ was added dropwisewith stirring at 0° C. Stirring was continued at 0° C. for 3 more hoursand then at room temperature overnight. The mixture was poured into 100ml. of pH 7.0 phosphate buffer solution and the solution was extractedwith CH₂ Cl₂. Evaporation of the CH₂ Cl₂ yielded pure cephalotaxyltrichloroethylcarbonate. The yield was 96% based on cephalotaxine used.

                  Table I                                                         ______________________________________                                               Isolated                                                                      yield,             High resol. MS                                      Example  %.sup.a  Formula     M.sup.+ calc.                                                                        M.sup.+ obsd.                            ______________________________________                                        1        71       C.sub.22 H.sub.25 NO.sub.7                                                                415.163                                                                              415.162                                  2        68       C.sub.23 H.sub.25 NO.sub.7                                                                427.163                                                                              427.162                                  3        83       C.sub.24 N.sub.27 NO.sub.7                                                                441.179                                                                              441.179                                  4        36       C.sub.24 H.sub.27 NO.sub.5                                                                409.189                                                                              409.188                                  5        91       C.sub.29 H.sub.28 NO.sub.8 Cl.sub.3                                                       .sup.b .sup.b                                   6        96       C.sub.21 H.sub.22 NO.sub.6 Cl.sub.3                                                       491.048                                                                              491.047                                  ______________________________________                                         .sup.a Based on cephalotaxine; not optimized.                                 .sup.b M.sup.+ -191 (--C.sub.3 H.sub.2 O.sub.3 Cl.sub.3 group); calc.         432.179, obsd. 432.181.                                                  

Chemotherapeutic acitivity of each of the compounds prepared in Example1-6 was determined in mice which were implanted with lymphocyticleukemia cells of the strain P388, according to the National CancerInstitute Protocols [Geran et al., Cancer Chemother. Rep., Part 3, 3:9(1972)]. Starting 24 hours after the tumor implantation, previouslydetermined dosages of each compound were injected intraperitoneally oncea day for 9 days. The results are shown in Table II. Survival time oftreated leukemic mice is compared to that of untreated leukemic mice(T/C×100). A T/C value of 100% indicates no activity. A T/C valuegreater than 100% means that the treated mice are surviving longer thanthe control mice. A compound giving a T/C value greater than or equal to125% is indicative of activity as defined by the NCI Protocols, supra.

                                      Table II                                    __________________________________________________________________________    Biological Test Data for Activity of Cephalotaxine                            Esters Against P388 Lymphocytic Leukemia in Mice                                                                   Animal weight                                                    Vehi-                                                                              Dose    difference                               Example                                                                             R Group           cle.sup.a                                                                          mg./kg./inj..sup.b                                                                    T-C      T/C.sup.c,                      __________________________________________________________________________                                                  %                               1A                      D    20      0.1      135                                    ##STR13##        D    20      -0.2     211                             C                       D    13      -0.8     154                             D                       T    20      -0.1     129                             2A                                                                                   ##STR14##        B    80      -0.7     145                             B                       B    40      -0.9     134                             C                       B    20      -1.0     125                             D                       B    10      -1.2     136                             E                       D    4.4     -1.5     147                             F                       D    1.9     0.5      134                             3A                      D    365     -3.0      198.sup.d                      B                                                                                    ##STR15##        D    240     -1.4     169                             C                       D    160     -0.1     183                             D                       D    160     -1.1     167                             E                       D    80      -0.1     173                             F                       D    40      -1.3     135                             4A                      D    80      0.9      150                             B                                                                                    ##STR16##        D    40      -1.8     125                             C                       D    20      0.9      130                             5A                      A    320     -2.3     136                             B                       A    160     1.2      154                             C                                                                                    ##STR17##        A    80      -1.0     138                             6A                      D    320     -1.0     172                             B                                                                                    ##STR18##        D    160     -0.9     162                             C                       D    160     -0.3     155                             D                       D    80      -0.9     183                             E                       D    80      -1.3     160                             F                       D    40      -0.4     140                             G                       D    40      0.9      183                             H                       D    20      -2.7     128                             I                       D    20      -1.5     160                             J                       D    20      -1.0     195                             K                       D    13      -0.5     138                             L                       D    8.8     -0.3     170                             __________________________________________________________________________     .sup.a A = saline, B = water + alcohol + acetone, C = water + acetone, D      water + alcohol, T = saline + Tween 80.                                       .sup.b One intraperitoneal injection per day for 9 days; DBA/2 mice.          .sup.c T/C = meansurvival time of test animals/mean survival time of          control animals; 125% or above considered active. Unaccountable variation     in T/C values among duplicate tests were sometimes observed; these may        possibly be due to solubility properties of the esters in the vehicles        used.                                                                         .sup.d One 30-day cure was reported.                                     

Dose levels other than those indicated in Table II were tested but gaveeither toxic or inactive responses. The terms "effective amount" and"effective dose" as referring to the treatment of animals is definedherein to mean those quantities of cephalotaxine ester which will causeremission of leukemia in the animal to which it is administered, withoutimparting a toxic response. The effective amount will vary with theparticular esters, the injection vehicle, the strain of leukemia, andother related factors. Generally for the instant esters, an effectivedose will be in the range of about 1.5-380 mg./kg. of body weight/day.The preferred dose range for a given ester is defined by the lowest andhighest dose shown in Table II for that ester.

The activities of these novel cephalotaxyl esters are not predictable,and in some cases are totally unexpected from structure-activitycorrelations based upon the naturally occurring active esters andcertain other synthetic esters. For instance, all of the naturallyoccurring active esters, such as harringtonine, homoharringtonine, anddeoxyharringtonine, contain a dicarboxylic acyl group and a tertiaryhydroxy group alpha to the carboxyl esterified with the cephalotaxinemolecule. The hydroxyl group is notably absent in all of our compounds,and the trans,trans-sorbate of Example 4 and the trichloroethylcarbonateof Example 6 do not exhibit the dicarboxylic acid moiety. Eighteen otheracyl esters, synthesized at approximately the same time as the subjectacyl esters, were inactive. For example, the cephalotaxyl ester ofmandelic acid, without the trichloroethylcarbonate moiety appearing inthe compound of Example 5, had no activity. In addition, toxicityobserved during administration of the mandelic acid ester at doses above80 mg./kg./injection is not present with the trichloroethylcarbonatemandelic acid ester at doses up to 320 mg./kg./injection. On the otherhand, the trichloroethylcarbonate of cephalotaxyl α-hydroxy-α-methylbutyrate is inactive. Likewise, the ethyl carbonate, benzyl carbonate,and chloro acetate esters of cephalotaxine are inactive. The sameunpredictability occurs with compounds related to the itaconate ester ofExample 3 by virtue of having α,β-unsaturation or α,β-unsaturationcoupled with a second carboxyl group. Thus, the methyl2-methyl-2-butendioate, methyl muconate, acrylate, methacrylate,cinnamate, and p-nitrocinnamate esters of cephalotaxine are allinactive.

It is understood that the foregoing detailed description is given merelyby way of illustration and that modification and variations may be madetherein without departing from the spirit and scope of the invention.

We claim:
 1. Chemotherapeutically active alkaloid compounds having thefollowing structure: ##STR19## where R is selected from the groupconsisting of: ##STR20##
 2. The alkaloid compound as described in claim1 wherein R is ##STR21##
 3. The alkaloid compound as described in claim1 wherein R is ##STR22##
 4. The alkaloid compound as described in claim1 wherein R is ##STR23##
 5. The alkaloid compound as described in claim1 wherein R is ##STR24##
 6. The alkaloid compound as described in claim1 wherein R is ##STR25##
 7. The alkaloid compound as described in claim1 wherein R is ##STR26##